Proximity switch



Feb. 21, 1967 D. TANN 3,305,805

PROXIMITY SWITCH Filed Nov. 14, 1963 M455 22 4 s F:

a /a H2 INVENTOR 774w ORNEYIS United States Patent 3,305,805 PROXIMITY SWITCH David Tami, 333 Covington Road, Detroit, Mich. 48203 Filed Nov. 14, 1963, Ser. No. 323,637 Claims. (Cl. 335-453) The present invention relates to a sensing device and more particularly, to a magnetically operated device for the detection of ferrous metals.

Magnetically operated switches which normally consist of a pair of magnetically permeable strips of metal spring biased in an open position and capable of closing under the influence of a magnetic field, are well known in the art. These switches, commonly known as reed switches, are capable of a variety of uses. Usually, this switch is connected into a utilization circuit of some type and is actuated by the introduction of a permanent magnet, the magnetic field of which causes the reed contacts to close, thereby completing the circuit of which the reed switch is an integral portion. In some cases an electromagnet is used to provide the lines of magnetic flux necessary to close the reed contacts of the switch.

As could be expected, the reed switch and permanent magnet can be combined in a variety of ways to provide switching devices of different types. For instance, they can be arranged along with a ferrous metal article, the latter being adjacent the magnet to absorb the magnetic flux and thus allowing the reed switch contacts to remain open. Upon the removal of the ferrous metal article the magnetic flux from the magnet is diverted toward the switch which then causes the contacts to close, thus pro- 'viding an indication of an absence of an article. In this instance, the combination is used as a sensing device operative upon the removal of a ferrous material.

The above combination of a reed switch and a permanent magnet can also be utilized to provide an indication of the existence of an adjacent ferrous material. When arranged in this manner the switch contacts are normally closed due to the magnetic field of the permanent magnet, but will open upon the insertion of a ferromagnetic object adjacent the permanent magnet which absorbs the magnetic flux, thereby allowing the reed contacts to separate due to the spring bias urging them normally apart. Operated in this manner the combination provides a proximity limit switch. Because of their ability to sense without contact and their easy mounting and wiring, proximity limit switches are very often inserted in machine tool beds and ways to limit the movement of slides, and also have many other applications such as checking devices for dimensional control, insuring closure of doors and safety gates, and in all kinds of indexing machines as a part detector. 7

However, in most of these proximity limit switches, a ferrous mass must pass very close to the permanent magnet before the magnetic force holding .the contacts together can be cancelled. For example, in some proximity switches the ferrous mass is not sensed until it is within .015. One method of achieving increased sensitivity in a magnetic reed contact device involves the utilization of a second or b-iasing magnet. When a single magnet is used to operate a reed switch, the magnet must be brought to a distance close enough to the reed switch to provide sufficient lines of flux to close .the contacts. When the magnet is pulled away from the reed switch the contacts normally stay closed until the magnet is at a distance much further from the reed switch than was required to originally close the contacts. It is upon this feature of the combination that the use of a biasing magnet depends. The biasing magnet is placed about midwaybetween the make" and break distances to provide a magnetic field that. keeps Patented Feb. 21, 1967 the contacts just closed. When an opposing magnetic field is introduced, the contacts open due to the neutralization of the opposing magnetic field. Since the reed contacts were biased at the point where .they were just closed, it can be seen that a more sensitive switch is provided. If the bias magnet was originally positioned so that the contacts were slightly apart, the introduction of an aiding magnetic field will close the contacts more readily. Thus, by properly biasing a magnetic ree-d switch, the device 0 becomes quite sensitive, and can be operated 'both in the normally open and normally closed position.

The above two-magnet method of increasing the sensitivity of a magnetic reed switch, however, it not readily adaptable to proximity switch devices. The positioning of both magnets with regard to the reed contacts is extremely critical, and movement of either one a very slight distance would destroy the delicate balance of the magnetic fields which has been achieved. Also, the strength of the magnetic field of each of the magnets will change slightly in time, and again the delicate balance which has been achieved will be destroyed by a change of strength of either of the permanent magnets. Therefore, the use of .a pair of magnets in proximity switch devices is severely limited by these factors.

The present invention provides the increased sensitivity usually found in devices employing a reed switch and two magnets without the disadvantages normally present in such devices, and is much more stable in operation.

The invention briefly comprises a single permanent magnet having generally parallel spaced apart pole faces with a reed switch positioned adjacent to and between the pole faces. The portion between the pole faces of the permanent magnet can be considered to be divided into three regions. The reed switch, when adjacent to either end region of the permanent magnet is normally closed, but when placed adjacent to the middle region, the switch contacts are normally open. Depending upon the position of the reed switch with regard to the permanent magnet, the combination is readily adaptable for use as a proximity switch with varying degrees of sensitivity.

When a reed switch is placed at the midpoint of the surface of a permanent magnet between the north and south pole faces, the contacts of the reed switch will remain open. As the reed switch is moved towards either pole, a point is reached where the contacts will close and remain closed until the pole face is reached. It is within these three regions on the surface of a permanent magnet between the opposite poles that .the present invention is concerned.

When the reed switch is placed at the midpoint between the opposite pole faces of the permanent magnet, and a ferrous mass passes within close proximity of either pole face, the normally open reed switch contacts will close. If now the reed switch is moved from the center point toward either pole face until the switch contacts close and then is backed off until they just open again, the device will operate somewhat differently. Assume that the reed switch was moved toward the south magnetic pole until the contacts close and then backed off slightly to a position where the contacts are again opened. The reed switch is still in the middle region but when a ferrous mass is moved toward the north pole, the switch contacts will close when the mass is spaced a much greater distance from the pole face than was the case when the reed switch was at the midpoint. Thus, greater sensitivity at the north pole is realized in this mode of operation, and it will be found that the switch has little or no sensitivity at the south pole face. If the reed switch is now moved in the middle region between the center point and the north pole face, a ferrous mass aproaching the south pole face will cause the contacts to close at a greater distance than when the switch was at the midpoint, and the switch will have little or no sensitivity at the north pole face.

As pointed out previously, when the reed switch is placed at the regions at both ends of the permanent magnet, the reed contacts will be normally closed. Assuming the reed switch is in the region adjacent the north pole face, which places it much further from the midpoint, a ferrous mass being moved toward the north pole face will cause the switch contacts to open at a distance further away than that which caused a reaction when the switch was at the midpoint. Again greater sensitivity is realized with little or no sensitivity at the south pole face. Finally, with .the switch in the region adjacent the south pole face, a ferrous mass introduced in the south pole face region will cause the reed contacts to open, while a ferrous mass in the vicinity of the north pole face will have little or no effect upon the contacts.

Accordingly, a principal object of the present invention is to provide an improved sensing device.

Another object of this invention is to provide a highly sensitive proximity switch device.

A further object of this invention is to provide a sensing device comprising a single magnet and reed switch.

Still another object of this invention is the provision of a sensing device which has the ability to sense ferrous metals without contact.

A still further object of this invention is to provide a proximity limit switch which consists of only one moving part.

A still further object of this invention is the provision of a sensing device which will sense the presence of ferrous masses at a distance greater than heretofore possible with a single permanent magnet and a reed switch.

Still another object of this invention is to provide a proximity switch which is of small size and high relia- 'bility.

- Still another object of this invention is to provide a sensing device which is capable of fast, positive operation.

Further objects and advantages of the present invention will become readily apparent as the following detailed description of the invention unfolds and when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a permanent magnet and magnetic reed switch illustrating features of the invention;

FIG. 2 shows one embodiment of the invention;

FIG. 3 is a'side view of the embodiment shown in FIG. 2;

FIG. 4 shows a second embodiment of the invention;

FIG. 5 is a cross-sectional view of the embodiment shown in FIG. 4 taken along the lines 5-5;

FIG. 6 is a cross-sectional view of the embodiment shown in FIG. 4 taken along the lines 66;

FIG. 7 shows another arrangement of a permanent magnet and magnetic reed switch combination; and

FIG. 8 is a circuit diagram of the electrical circuit used in conjunction with the present invention.

For a better understanding of the invention reference is made to FIGURE 1 which shows a combination of a permanent magnet 11 and a conventional magnetic reed switch 12. Magnetic reed switch 12 consists of a pair of magnetic reeds or ferrous metal strips 13 and 14, with terminal leads 16 and 17 located outside of glass envelope 18 and providing for connection to an external circuit to be controlled by the reed switch 12 as will be described. The magnetic reeds 13 and 14 spring bias themselves apart so that the switch 12 is normally open.

Permanent magnet 11, shown in FIGURE 1 as being of a rectangular shape, can be of any size or shape which will best suit the purpose of the intended use of the device. Permanent magnet 11 is of the usual type having north and south pole faces, designated in FIGURE 1 as N and S, respectively. The lower surface 15 of permanent magnet 11 can be considered as being divided into three operating regions, namely those between A-B, BC, and C-D. The midpoint of the distance between the north and south pole faces has been designated by E, and the region between BC is referred to as the middle region.

With reed switch 12 adjacent to midpoint E, as shown in FIGURE 1, the magnetic reeds 13 and 14 are apart despite the fact that the end of the switch is as near to the magnet 11 as it can get. If a ferromagnetic object or any ferrous mass 11a, is brought into adjacent relationship to either the north or south pole faces, magnetic reeds 13 and 14 will come together or close. The distance from the pole faces at which this takes place depends to a certain extent on the size of the permanent magnet 11, its magnetic field strength, the size of the ferrous mass and the size of the reed switch which is used. Using an average strength magnet having pole faces of approximately square, and a thickness of approximately 7 a reed switch having a length of /s closed when a ferrous mass came within approximately of either the north or south pole faces.

If reed switch 12, which has its longitudinal axis perpendicular to the distance between the pole faces, is moved to the right near C which is approximately midway between midpoint E and D as shown in FIG. 1, but still between midpoint E and C, a ferrous mass coming within about double the distance previously mentioned of the north pole face will cause the contacts 13 and 14 to close. The same mass, however, has little or no effect on the south pole face. As can be expected, the reverse is true when the reed switch 12 is placed in the equivalent position between midpoint E and B, which is approximately mid-way between midpoint E and A. Under such circumstances a ferrous mass brought within about double the distance when reed switch 12 was at midpoint E of the south pole face will cause the contacts to close, while the same ferrous mass placed against the north pole face will have little or no effect.

When the magnetic reed switch 12 is positioned in the region between C and D, the magnetic reeds 13 and 14 will close. When a ferrous material is now brought into proximity with the south pole face, the reed switch will open when the ferrous mass is at a greater distance than noted above. For instance, using the same magnet and reed switch as cited in the previous examples, when the ferrous mass came within more than double the distance first mentioned of the south pole face, metal strips 13 and 14 opened. The same ferrous mass could be brought up to the north pole face with little or no effect on the closed reed switch 12. Similar operation results when reed switch 12 is put in a region A-B. A ferrous mass brought in proximity to the north pole face causes the switch reeds 13 and 14 to open, and the ferrous mass brought to the south pole face of permanent magnet 11 has little effect.

As can be noted from the above description, there has been provided a highly sensitive sensing device which can employ a reed switch having normally open contacts and be operated from either of two faces with average sensitivity; employ a reed switch having normally open contacts and be operated from one face with greater sensitivity; and employ a reed switch having normally closed contacts which can be operated from one face with very high sensitivity. It should be noted also that under the latter two types of operations just described, the position of the reed switch adjacent to the surface of the permanent magnet can be changed to obtain the high sensitivity operation off the other pole face, if desired.

In all of the above described positions of the reed switch 12, the magnetic reeds 13 and 14 are biased by the north and south pole faces so that they will be opened and closed by the ferrous mass at distances from the pole faces which are close together. In addition the repeatability of the switch is good. That is the magnetic reeds 13 and 14 will open and close at substantially the same: point each time a ferrous mass is moved toward and away from a pole face. In contrast with this, a reed switch actuated in a conventional manner by moving a magnet.

toward and away from the switch is very sluggish and has poor repeatability. That is the points at which the magnet will open and close the switch are widely spaced, and the switch does not open and close at the same points each time. p

In FIGURES 2 and 3 there is shown one practical embodiment of the invention. FIGURE 2 is a front view of what may be termed a lollipop configuration comprising a magnet 19 and a holder 21. Magnet 19 in this instance is in the shape of a thick disc with the north and south pole faces on the ends thereof. Holder 21, which can be made of any suitable non-magnetic material, has a circular portion 22 which surrounds the periphery of the magnet 19 leaving both the north and south pole faces open, and also includes a handle 23 having a reed switch 12 embedded therein. Also shown are a pair of leads 26, 27 which are connected to terminal leads 16 and 17 (FIGURE 1), respectively.

Referring now to FIGURE 3, it can be seen that the end of the reed switch 12 is at the midpoint of the north and south pole faces of disc magnet 19. As pointed out above, the reed switch contacts will be normally open with this arrangement and will close when a ferrous mass comes into proximity of either pole face.

FIGURES 4-6 illustrate a second embodiment of the invention providing a sensing device which can be adjusted. The device comprises a cylindrical permanent magnet 28 in a non-magnetic holder 29. Permanent magnet 28 is arranged so that its north and south pole faces (N and S) are left uncovered as in the embodiment of FIGS. 2 and 3. Holder 29 comprises an upper portion 31 which encircles permanent magnet 28 thus holding it in place, and a lower portion 32 having elongated chambers 33, 34, and 35 into which a reed switch 12 can be easily inserted. Chamber 33 is aligned with the operating region E-C, chamber 35 is aligned with the operating region CD, and chamber 34 provides for operation in the region adjacent to midpoint E, described above with regard to FIGURE 1. A cover plate 37 is fastened to the bottom of the lower portion 32 to hold the reed switch positioned in any of the chambers in place, and contains small apertures (not shown) for allowing the lead lines of the reed switch to project therethrough.

A reed switch placed in chamber 33 will have its contacts normally open and upon the introduction of a ferromagnetic object in proximity to the north pole face, the contacts will close to indicate the presence of the object. If the reed switch is removed from chamber '33 and placed in chamber 35, the reed switch contacts will be normally closed and upon the introduction of a ferrous mass near the south pole face of permanent magnet 28 the contacts will open. If operation is desired off both faces, the reed switch may be removed from chamber 35 and placed in chamber 34 thereby providing operation at the midpoint of permanent magnet 28. The contacts of the reed switch in this mode of operation will be normally open and when a ferromagnetic object is within close proximity of either the north or south pole faces the contacts will close. In this manner there is provided a sensing device comprising a permanent magnet which is readily adjustable and can operate with great sensitivity off either of its pole faces, or with a lesser degree of sensitivity off both its pole faces.

The above embodiment utilizes only three reed switch chambers. If greater versatility is desired, a sensing device having 5 chambers can easily be provided. Such a device would allow operation with a permanent magnet in the regions A-B, BE,' E, E-C, and C-D. This device would then utilize the five useful regions of operation along the surface of a permanent magnet between its pole faces as described in connection with FIG. 1.

FIGURE 7 shows another arrangement of reed switch 12 and permanent magnet 11 providing the same operation as described in FIGURE 1. In this case, however, the longitudinal axis of reed switch 12 is parallel rather than perpendicular to the surface 15 between the north and south pole faces of permanent magnet 11, and is parallel to the axis between both faces. Magnetic reeds 13 and 14 will remain closed in the regions A-B and C-D while remaining open in the region B-C. A ferrous mass brought into proximity with the north or south pole faces will provide identical results as previously described in conjunction with FIGURE 1.

Also, reed switch 12 can be operated with identical results by placing its longitudinal axis again parallel to surface 15 but perpendicular to the axis between the north and south pole faces. In this mode of operation the switch is rotated from the position described in FIG. 7 in a plane parallel to that describing surface 15.

In FIGURE 8 there is shown a circuit diagram for use with the present invention. A normal llO-volt, 60.- cycle A.C. voltage (not shown) is connected across the input terminals 38 and 39 providing a voltage across primary winding 43 of a transformer 42. A switch 41 is used to actuate the circuit. Secondary winding 44 provides voltage through a rectifier 46 to the remainder of the circuit. Capacitor 47 is utilized to remove any ripple voltage from the rectified DC. voltage.

A transistor 52 having a base 53, emitter 54, and collector 55 operates a relay 57 which comprises a coil 58, stationary contact 59 and a movable contact 60. A resistor 48 is included in the emitter port-ion of transistor 52, while a second resistor 49, in the collector network of transistor 52, provides an added resistive load in that portion of the circuit. Resistors 50 and 68 are included as current limiting resistors in that portion of the circuit to which a reed switch is connected. The reed switch 12 is shown connected between reed switch terminals 66 and 67 by dotted lines. A diode 64 is connected across coil 58 of relay 57 to provide a discharge path for the high voltage that builds up on coil 58 and thus helps protect the transistor 52. Output terminals 62 and 63 are connected to stationary contact 59 and movable contact 60, respectively, and can be connected to an indicator or other operating circuits.

In operation, the circuit is energized when the contacts of switch 41 in the primary circuit of transformer 42 are closed. Rectifier 46 rectifies the A.C. voltage appearing in secondary winding 44 thus providing a DC. voltage for transistor 52. Without a reed switch connected across reed switch terminals 66 and 67 transistor 52 is in its normal non-conductive state and relay 57 is in its unenergized position with contacts 59 and 60 apart. Assuming now that reed switch 12 is connected across the terminals 66 and 67 the circuit will remain in the same state while the magnetic reeds in the reed switch remain apart. When the magnetic reeds of reed switch 12 come together, resistor 68 is put into parallel with resistor 50 thus increasing the current flowing in base 53 of transistor 52. This causes transistor 52 to be turned on, increasing the current in collector 55, resistor 49, and coil 58 of relay 57. The increased current causes contacts 59 and 60 to close, completing the circuit through output terminals 62 and 63'. Thus it can be noted that there is provided a fast-operating, lowpower switching circuit.

When the magnetic reeds of reed switch 12 open, resistor 68 is removed from the base network, thus decreasing the amount of current in base 53 which in turn causes transistor 52 to return to its non-conductive state. With the transistor being turned off, the current in collector 55 decreases quickly as does the current in coil 58 and resistor 49. The magnetic pull is thus removed from movable contact 60 which is normally spring biased away from stationary contact 59, allowing movable contact 60 to return to its normal open position breaking the circuit to which output terminals 62 and 63 are connected. The sudden decrease of current through coil 58 can cause a high inductive voltage surge that can possibly damage transistor 52, which is countered by the addition of diode 64 across coil 58 providing a discharge path for this voltage. The circuit is now in its normal state with transistor 52 being turned off and the contacts 59 and 60 of relay 57 remaining open.

It can now be seen that there has been provided an electrical circuit which allows a variety of uses where quick-acting, high voltage and current applications are desired. The circuit operates on normal llO-volt, 60- cycle AC. voltage, and because of the low current carrying capabilities of the magnetic reeds of a normal reed switch, is necessary in most applications.

From the foregoing, it will be seen that the present invention provides a sensing device comprising a permanent magnet, reed switch, and simple circuit, which provides the user an ability to sense ferromagnetic objects at a greater distance than heretofore possible. The device is also extremely accurate, capable of fast operation, small and compact to allow for easy mounting and wiring, extremely reliable, and has high power-handling capacity. It is also readily adapted to be operated from either or both of its sensing surfaces with only a minor adjustment.

The above description is of preferred embodiments of the invention and many modifications may be made thereto without departing from the spirit and scope of the invention, which is defined in the appended claims.

What is claimed is:

1. A magnetically operated sensing device for detecting the presence of a ferrous mass comprising a housing, a magnet mounted in said housing having a north and a south pole face, and a reed switch, said housing having a plurality of elongated chambers each adapted to retain one end of said reed switch substantially against a surface of said magnet at different positions between said pole faces, said reed switch being positioned in one of said chambers, said reed switch being act-uatable from an open to a closed position by a ferrous mass moved into proximity with one of said pole faces, when positioned in another of said elongated chambers said reed switch being actuatable from a closed to an open position by a ferrous mass moved into proximity with one of said pole faces.

2. A proximity switch for sensing the presence of a ferrous mass comprising a reed switch having normally open contacts, a permanent magnet having a north and a south pole face, wherein the outer surface of said mag;- net extending along the axis of said pole faces is defined by a first pair of magnetic regions between the midpoint of said pole faces and a position approximately midway between said midpoint and each pole face, and a second pair of magnetic regions between said mid-way positions and each of said pole faces, and means for retaining said reed switch substantially against the surface of said magnet in one of said first pair of magnetic regions and displaced from said midpoint whereby the normally open contacts of said reed switch are maintained opened but are closed by a ferrous mass moved into proximity of the pole face furthest from said positioned reed switch.

3. A proximity switch for sensing the presence of a ferrous mass comprising a reed switch having normally open contacts, a permanent magnet having a north and a south pole face, wherein the outer surface of said magnet extending along the axis of said pole faces is defined by a first pair of magnetic regions between the midpoint of said pole faces and a position approximately mid-way between said midpoint and each pole face, and a second pair of magnetic regions between said mid-way positions and each of said pole faces, and means for retaining said reed switch substantially against the surface of said magnet in one of said second pair of mag- .netic regions whereby the normally opened contacts of said reed switch are maintained closed but are opened by a ferrous mass moved into proximity of the pole face nearest to said positioned reed switch, and the normally open contacts are relatively insensitive to movement of the ferrous mass into proximity with the pole face furthest from said reed switch.

4. A proximity switch for sensing the presence of a ferrous mass comprising a reed switch having normally opened contacts, a permanent magnet having opposed north and south pole faces, the surface of said magnet being defined by a middle magnetic region and a pair of end magnetic regions, wherein the magnetic field of said magnet maintains the reed switch contacts open when said reed switch is positioned adjacent to the surface of the middle region and maintains the reed switch contacts closed when said reed switch is positioned adjacent to the surface of either of said end regions, and means for retaining one end of said reed switch substantially against the surface of said middle magnetic region adjacent to the boundary of one of said end regions, whereby said reed switch is closed by a ferrous mass moved into proximity with the pole face furthest from said boundary.

5. A proximity switch for sensing the presence of a ferrous mass comprising a reed switch having normally opened contacts, a permanent magnet having opposed north and south pole faces, the surface of said magnet being defined by a middle magnetic region and a pair of end magnetic regions, wherein the magnetic field of said magnet maintains the reed switch contacts open when said reed switch is positioned adjacent to the surface of the middle region and maintains the reed switch contacts closed when said reed switch is positioned adjacent to the surface of either of said end regions, and means for retaining one end of said reed switch substantially against the surface of one of said end magnetic regions, whereby said reed switch is opened by a ferrous mass moved into proximity with the pole face closest to said positioned reed switch end region.

6. A proximity switch for sensing the presence of a ferrous mass comprising a reed switch magnetically actuatable between open and closed positions, a magnet having a north and [a south pole face, means retaining said reed switch against movement relative to and in a predetermined asymmetrical position relative to the lines of flux produced by said pole faces to produce biasing lines of flux through the reed switch to bias said reed switch in one of said positions, said proximity switch being sensitive to the presence of the ferrous mass adjacent to one of said pole faces to actuate said reed switch to the other of said positions and being relatively insensitive to the presence of said ferrous mass adjacent to the other of said pole faces.

7. The proximity switch as defined in claim 6 wherein said reed switch is biased in a normally open position and actuated to said closed position when the ferrous mass is moved adjacent to said one pole face.

8. A proximity switch as defined in claim 6 wherein said reed switch is biased to said closed position and actuated to said open position when the ferrous mass is moved adjacent to said one pole face.

9. The proximity switch as defined in claim 6 wherein said north and south pole faces are generally parallel to one another and define an axis therebetween with said reed switch positioned substantially parallel to said axis.

10. A proximity switch for sensing the presence of a ferrous mass comprising a magnet having a north and south pole face disposed generally parallel to one another and defining an axis therebetween extending from one pole face to the other, a reed switch magnetically actuatable between open and closed positions, said reed switch being longer than the distance between said pole faces and being positioned adjacent to said magnet parallel to said axis with one end of the reed switch disposed between said pole faces and the other end of the reed switch projecting beyond one of said pole faces, means retaining said reed switch against movement relative to and in a predetermined asymmetrical position relative to the lines of flux produced .by said pole faces to produce biasing lines of fiux through the reed switch to bias said reed switch in one of said positions, said reed switch being actuatable to the other of said positions when the ferrous mass is moved adjacent to the other of said pole faces.

11. A proximity switch as defined in claim 10 including solid state amplifying circuit means switched between ON-OFF conditions by said reed switch.

12. A proximity switch as defined in claim 6 including solid state amplifying circuit means switched between ON-OFF conditions by said reed switch.

13. The proximity switch as defined in claim 6, including means for adjusting said predetermined asymmetrical position of said reed switch relative to the lines of flux produced by said pole faces.

14. The proximity switch as defined in claim 6 wherein said retaining means retains one end of said reed switch substantially against a surface of said magnet between said pole faces with the longitudinal axis of said reed switch perpendicular to said surface.

15. The proximity switch as defined in claim 6 wherein said north and south pole faces are at opposite ends of 10 said magnet and define an axis extending from one pole face to the other, and wherein said retaining means retains one end of said reed switch substantially against l3. surface of said magnet between said pole faces with the longitudinal axis of said switch parallel to the axis defined by said pole faces.

References Cited by the Examiner UNITED STATES PATENTS 2,319,937 5/ 1943 Mathes 200-87 2,322,851 6/ 1943 Kalb 200-87 2,431,319 11 1947 Ellwood 200-87 2,945,928 7/ 1960 Houser 200-87 3,009,033 11/1961 Werts 200-87 3,011,036 11/ 1961 LaRocca 200-87 3,020,369 2/ 1962 Jacobson 200-87 3,164,696 1/ 1965 Pusch 200-87 3,168,269 2/1965 Fisher 200-87 3,185,974 5/ 1965 Doane 200-87 3,199,093 8/1965 Cheney 200-87 3,205,323 9/1965 Deshautreaux 200-87 BERNARD A. GILHEANY, Primary Examiner. B. DOBECK, I. BAKER, Assistant Examiners. 

1. A MAGNETICALLY OPERATED SENSING DEVICE FOR DETECTING THE PRESENCE OF A FERROUS MASS COMPRISING A HOUSING, A MAGNET MOUNTED IN SAID HOUSING HAVING A NORTH AND A SOUTH POLE FACE, AND A REED SWITCH, SAID HOUSING HAVING A PLURALITY OF ELONGATED CHAMBERS EACH ADAPTED TO RETAIN ONE END OF SAID REED SWITCH SUBSTANTIALLY AGAINST A SURFACE OF SAID MAGNET AT DIFFERENT POSITIONS BETWEEN SAID POLE FACES, SAID REED SWITCH BEING POSITIONED IN ONE OF SAID CHAMBERS, SAID REED SWITCH BEING ACTUATABLE FROM AN OPEN TO A CLOSED POSITION BY A FERROUS MASS MOVED INTO PROXIMITY WITH ONE OF SAID POLE FACES, WHEN POSITIONED IN ANOTHER OF SAID ELONGATED CHAMBERS SAID REED SWITCH BEING ACTUATABLE FROM A CLOSED TO AN OPEN POSITION BY A FERROUS MASS MOVED INTO PROXIMITY WITH ONE OF SAID POLE FACES. 